ducatipaso.org

Mctool
The thinking man

Yes,something to work on,problem is jets are getting hard to come by these days

wadda ya think Jay?

Your a friggin mind reader Mctool..... I have been giving it plenty of thought lately,as even the slightest bit of extra fuel on small throttle openings is too much for my bike in my climate,but on harder acceleration, when needed ,the extra fuel would go out the pump jet instead of the bypass.There are also many jets available from other manufacturers so once a suitable spot to locate the bypass was found it would be a case of drill a hole tap a thread and then start increasing the jet size until the stumble was eliminated.I'm not overly concerned about drilling a hole in my weber casing as it a fairly easy matter to blank it off again,if it doesn't work out.

I was initially worried about grinding too much off my cam and creating a lean spot on acceleration,because the stumble is so persistent,you start to wonder if a lean stumble is starting to occur,especially when your 1/3 way around the cam with no fuel being pumped and the stumble is still occuring,but now I have almost got it eliminated on one of my alloy cams,and you will laugh when you see how much is left of it.

I think the main problem is that even the slightest bit of pump stroke supplies way to much fuel at small throttle openings,so wherever you start to ramp the cam,unless it is an incredibly shallow ramp,you create a stumble,and it seems to move further and further up the rev range,the more you take off the cam.One advantage here is that the further open the throttle is the more air and the less likelyhood of a stumble from a too rich mixture.So eventually you eliminate it,but create a slack spot off idle by doing so.

The main benefit of the method Mctool is suggesting is that throttle opening speed dictates the amount of fuel delivered by the pump rather than a proportional amount of fuel supplied for any given throttle opening regardless of speed of opening.It is only on fast throttle openings that the extra fuel is needed,and this doesn't effect the performance of my bike at all.It is only on gentle openings that problems occur so...

First things first,a series of good accurate drawings of the dcnf carb would have to be examined and a list of likely positions evaluated,then based on the best position,an idle jet or something similar sourced that will fit and is cheap and readily available.It would be nice to be able to access the bleed easily(without removing the carb top cover),and also from the point of view of making an easy modification for others to do,perhaps beneath or beside the pump cover,would be a good place to start looking.


I have read of others modifying carbs similarly so I have no doubts that it could be done,and with a little ingenuity,a really simple solution to the dreaded weber stumble could be fabricated.

So Yes Higgy I think that it is definately worth looking at ,Mctool has been taking too many anabolic smartoids again,and I think that this time he has hit the nail right on the head.

Just one last thing while I'm thinking of it,My next experiment now that I have ground so much off the alloy cams was going to be totally removing the cam - no pump whatsoever,to see exactly how far up the rev range you can go without it.I'm not sure it's going to prove much,but wouldn't it be funny if it works fine without it ? J.

I love these little projects so heres a starter http://www.ebay.com.au/itm/WEBER-PUMP-E ... 439a0946fd as for location,how about in the middle of the pump spring retaining hole beneath the diaphragm ? I'm sure theres a better place but its a start. J.

I also have in regular use a weber 28/36 DCD which has a check valve between the float bowl and the accelerator pump inlet. It consists of a small ball berring ( about 2-2.5mm ) held down by gravity against the seat ( small brass insert ). Used to bother me that this valve always seemed kinda leaky ..... not any more AND I would think that over the passage of time this ball berring would wear the seat thus reducing the size of the orifice, assuming the leakage was caused by a delibratly misformed seat.
anabolic smartoids ? yeah Homer takes them I just have Oxy

The DCNF has one of those check valves fitted too,its the inlet to the pump,if it aint working,it squirts the fuel back into the float chamber instead of out the nozzles.same principle a little ball that seats and stops the fuel returning. J.

just checked the haynes manual and according to it,the valve also has a calibrated discharge hole in its brass housing,in theory you could just enlarge this hole ?,but I think you would have some dramas if you stuffed it up as I'm not sure if they are available still.

Are you sure your not on the smartoids Mctool ?

The brass insert in the DCD doesnt look at all replacable ..... sort of looks cast in .
I think the issue here is that the accelerator pump discharge volume is probly about right , just that its over faster than your 1st %uck. As we stand at the moment the duration is pretty much as long as it takes you to yank the throttle open, and I cant see a way of lengthening this shot ( I think it appears that the shot needs to continue after the throttle has reached full open) ..... which brings us back to the reserve volume and the access to it ( emulsion tubes ) which would need to be working properly ( delivering the correct amount of correctly emulsified fuel ) before the pump shot is over..... they aint.
Somewhere between "Weberology and this accelerator pump thread lies the answer
Jayh Ive always been clever , just that people do get sick of some one who is clever all the time and it pays to keep something up your sleeve

At the risk of repeating myself and sounding too clever (which Mctool tells me is not going to help my popularity ) This is what I now believe is causing all of the problem with my weber.

I don't believe that any of the missing that the 750 experiences is caused by a lean spot.

Even in top gear at 120 kmh,at a steady cruise,the throttle is only about 7 degrees open (when measured at the twist grip).The thing to keep in mind here is that the throttle butterflies are only just barely open at this stage,so the engine is getting enough air/fuel mix at the correct ratio to propel you along in top gear with the throttle butterflies only open a crack.

If you give it a fist full of throttle at this stage,it just gets up and goes,plenty of fuel available and the pump is doing its job but the important thing is you are also supplying plenty of extra air as you twist the throttle an extra 10 - 20 degrees.It's only when you just touch the throttle lightly that the missing occurs.Just a tiny twist of the throttle moves the diaphragm enough to deliver a massive dose of extra fuel,enough indeed for a far greater load,a car,(which because of its mass would have the throttle open much further to propel it along at cruising speed).For a split second this makes the mixture go way rich.

When the engine is warmed up fully it just cant handle it.Plenty of times while the bike has been warming up I've thought the problem was gone,it may take 1/2 hour of riding before a stumble begins to creep back in.The webers really change a lot as they warm up,especially down low,the idle circuit is a good example of this,and can only properly be set after a good ride.I think that people in a colder climate may experience less of a problem with their webers as they never heat up to the same degree as those in a hot climate.

I think that the valve on the dcnf will be hard to replace too Mctool,but if a drill can be inserted down the bypass galley and a larger bypass hole drilled in the casing,this may be a really simple solution to the problem.Perhaps a thread could be tapped in the end of the galley and a replaceable jet fitted ?

Once you get rid of this large dose of fuel from the pump,the webers start to respond to all of the tuning changes as per the text book,Most of the stock 750 jetting is a compromise that trys to work in with the excess of fuel from the pump.

I am really confident that your ideas can be turned into reality and the resulting webered paso converted into a thing of great beauty Jay.

My weber is off the bike again and about to have major surgery performed on it.Not happy with the original diagnosis we received a second opinion and decided on a joint bypass operation / lobe reduction in an attempt to fix the murmur.

The pump diaphragm will be removed and a 4mm hole made through the centre of the spring retainer into the float chamber.This will house a dcnf main jet that has been modified for reduced flow to match the cam profile / pump capacity.Calculations have determined that 1mm of pump stroke pumps approx. .6 cc of fuel,which is about 4 times what the .35 jets can squirt in a second so based on that , a 1mm orifice will be used as a starting point.A blanked jet could be fitted to the hole to rectify any problems that occur, as a result of malpractice on the part of the surgeon.

Not being very well versed in fluid dynamics,my calculations are really no more than an educated guess,so fingers crossed she pulls through.....2000 - 3000 rpm smoothly. J.

fingers and toes
I really don't think there is a need for an experimental physicist here

just a good flow bench and some empirical measurements

but then I've been known to be wrong
from time to time..........ROTFLMAO!!!

Heres what I have done to the weber.

1st I blanked off all holes with masking tape,so no debris ended up in any of my jets (hopefully)
then I drilled a 4mm hole in three stages 2,3.5,4mm so as to get a neat hole.I positioned it in the centre of the diaphragm spring holder,It's off centre though ,but no one will see it in there.(I was reluctant to centre punch the cast material).
The wall of the float chamber is about 2.5 - 3mm thick here.



Next I thoroghly cleaned out all of the swarf from the drilling and test fit a dcnf main jet,which can be seen protruding through the wall of the float chamber just to the right and low of centre in this picture



And here it is viewed from the outside looking into the pump,the spring sits around the outside of it,and it sits below the height of the surrounding ring so the diaphragm doesn't touch it.



Now I can adjust the size of the bleed by substituting main jets.I was going to start with a 1 mm main but instead I soldered up a 145 and drilled it to .75mm. I fitted a cam that I knew was too rich and took it for a ride..... still to rich,but not bad,better than it was - I should have gone with the 1mm , anyway they are not hard to swap now and the pump seems to be working ok aside from the extra fuel so it appears to be a success as far as location of the bleed goes (I was a little bit concerned about fuel bleeding back while the pump is not in use,but it seems ok so far).I think that the whole diaphragm area is below fuel level by the looks of it.

Now the hard bit - trying to re-create a stock cam and matching a jet size to it.I think that a continually rising flat ramp will be the best profile for the cam (like stock),with a jet matched to bleed fast enough for any slow throttle movements to completely bypass the pump jets.This would mean that any slow openings regardless of the position on the cam would squirt no fuel into the carb.
In theory I should be able to increase the size of the bleed until the stumble just stops and that should be the right size !

If for some reason this all turns to shit ,I'll solder up a main jet and block the hole

one more pic of my 82 pantah 600 motor with paso transmission and f1 dry clutch fitted,it's for my ncr replica project and I'll get back to it when the weber is finished (one day soon hopefully !).



more tests later in the week,goin' fishin' this arvo' J.

it just occured to me that I can now visually verify when the bleed is doing it's job !

A quick visual check of the pump jets confirmed that I was now lean (no fuel squirting unless I really twisted the throttle quickly),instead of the rich condition I thought it was,so I reduced the jet size to .50 mm and visually checked there was fuel squirting when I gave the throttle a moderate twist.All good so off for another ride.
This time it ran well,pulling strongly in 3rd an 4th from as low as 2000 rpm.It won't do this when too rich.There was a slight stutter in 1st gear when accelerating moderately,this is most probably a rich miss from the pump,I'll wait until I go for a decent ride and see if it gets better or worse with heat.

I won't have a cam to modify back to original specs for a couple of weeks,so I won't be able to post any info on stock setups for a while but I did have a play with jetting and float levels and it appears at this stage that a large main jet is still required as well as a higher than stock float level to eliminate the lean spot up higher.
The setup I took for a ride was 57 idles (stock),170 mains,200 AC's,44 float level.This varied from the setup I've been riding for quite some time which was 55,170,185,44 float,I don't think it felt quite as responsive,but that could be the larger AC's or idles,most probably the idles or my imagination.

The .50 bleed is working well with my heavily modified pump cam,but the stock cam is far more aggressive so I'm not sure that it will be big enough for stock.The point of the exercise was to make the accelerator pump dose more controlled and as far as this goes it was a definite success .
the modification works as intended,is simple to do and does not impede pump operation other than in the intended way.I think that once the correct size bleed for the stock setup is found,a hole of the correct diameter and a mildly altered cam will be all that is required.Now to complete the tuning. J.

Hey Jay,
sorry to go off topic but I was wondering how you got you motor looking like it is.Did you get it bead blasted etc?.

Cheers
Andrew

Hi Andrew, I just used the spray can degreaser (cheap stuff 5 cans for $10) and a brass wire brush.I noticed one day after I had sprayed it on my conti race cans and hadn't wiped it off that it cleaned the aluminium where it had been wet and left the rest dull looking,so now I soak brass and aluminium in it for that new look.You have to keep it wet with the stuff for an hour or so,so you need to stop it evaporating in a plastic bag or something. J.

CLR and Marine clean also work well

jayh wrote:Hi Andrew, I just used the spray can degreaser (cheap stuff 5 cans for $10) and a brass wire brush.I noticed one day after I had sprayed it on my conti race cans and hadn't wiped it off that it cleaned the aluminium where it had been wet and left the rest dull looking,so now I soak brass and aluminium in it for that new look.You have to keep it wet with the stuff for an hour or so,so you need to stop it evaporating in a plastic bag or something. J.

Thanks Jay,I'll give it a go.Good work on the weber too mate

Cheers
Andrew